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If you are sitting in an ordinary chair, lean over and look at the ground directly below your head. A cylinder the diameter of your eye drawn from your eye to the ground would enclose about as many particles of air as there are interstellar particles in a cylinder of the same diameter but extending between our solar system and the center of our galaxy 27,000 light years away. Though the space between the stars is emptier than the best vacuums created
on the Earth (those are enclosed spaces devoid of matter, not the
household
cleaning appliances), there is some material between the stars composed of
gas and dust. This material is called the interstellar medium. The
interstellar medium makes up between 10 to 15% of the visible mass of the
Milky Way. About 99% of the material is gas and the rest is "dust".
Although the dust makes up only about 1% of the interstellar medium, it has
a much greater effect on
the starlight in the visible band---we can out see only roughly 6000 light years
in the plane of the Galaxy
because of the dust. Without
the
dust,
we
would
be
able
to
see
through
the
entire 100,000 light year disk of the Galaxy. Observations of other galaxies
are done by looking up or down out of the plane of the Galaxy. Dust provides
a place for molecules to form. Finally, probably the most of important of all
is that stars and planets form from dust-filled clouds. Therefore, let us look
at the dust first and then go on to gas. The structure of the Galaxy is mapped
from measurements of the
gas.

The dust is made of thin, highly flattened flakes or needles of graphite
(carbon) and silicates (rock-like minerals) coated with water ice. Each
dust
flake is roughly the size of the wavelength of blue light or smaller. The
dust
is probably formed in the cool outer layers of red giant stars and
dispersed in
the red giant winds and planetary nebulae.

Starlight passing through a dust cloud can be affected in a couple of ways.
The light can be totally blocked if the dust is thick enough or it can be
partially scattered by an amount that depends on the color of the light and
the thickness of the dust cloud. All wavelengths of light passing through a
dust cloud will be dimmed somewhat. This effect is called
extinction.

Dark dusk clouds like these "Bok globules"
in IC 2948 were once thought to be holes in the sky. The dark clouds block
the light from the emission nebula behind.

A dark cloud, Barnard 86, is silhouetted
against a starry background. Stars form in the dark clouds. A young
cluster, NGC 6520, probably associated with Barnard 86 is seen just to
the left of it.

The discovery of the dust is relatively recent. In 1930 R.J.
Trumpler (lived 1886--1956) plotted the angular diameter of star
clusters
vs. the distance to the clusters. He
derived the distances from inverse square law of brightness:
clusters farther away should appear dimmer.
IF clusters all have roughly the same
linear diameter L, then the angular diameter
q should
equal a (constant L) / distance. But he found a
systematic increase of the linear size of the clusters with
distance.

This seemed unreasonable! It would mean that nature had put the
Sun at a special place where the size of the clusters was the smallest. A
more
reasonable explanation uses the Copernican principle: the Sun is in a
typical
spot in the Galaxy. It is simply that more distant
clusters have more stuff between us and cluster so that they appear fainter
(farther away) than they really are. Trumpler had shown that there is dust
material between the stars! The extinction of starlight is caused by the
scattering of the light out of the line of sight, so less light reaches us.

Not all wavelengths are
scattered equally. Just as our air scatters the
bluer colors in sunlight more efficiently than the redder colors, the
amount of
extinction by the interstellar dust depends on the wavelength. The amount
of
extinction is proportional to 1/(wavelength of the light). Bluer
wavelengths
are scattered more than redder wavelengths.

The 1/l behavior of the scattering indicates
that the
dust size must
be about the wavelength of light (on the order of 10-5
centimeters). Less blue
light reaches us, so the object appears redder than it should. This effect
is
called reddening, though perhaps it should be called
"de-blueing".
If the dust particles were much larger (say, the size of
grains of sand), reddening would not be observed. If the dust particles
were much smaller (say, the size of molecules), the scattering would
behave
as 1/l4.
Trumpler showed that a given spectral type of star
becomes increasingly redder with distance. This discovery was
further
evidence for dust material between the stars.
If the Sun is in a typical
spot in the Galaxy, then Trumpler's observation means that more distant
stars have more dust between us and them.

You see the same effect when you observe the orange-red Sun close to the
horizon.
Objects
close to the horizon are seen through more atmosphere than when they are
close
to zenith. At sunset the blues,
greens, and some yellow are scattered out of your line of sight to the Sun
and
only the long waves of the orange and red light are able to move around
the air
and dust particles to reach your eyes.

At near-infrared (slightly longer than visible light) the dust
is transparent. At longer wavelengths you can see the dust itself glowing and
you can probe the structure of the dust clouds themselves as well as the stars
forming in them (young stars that are hidden from us in the visible band).
The Spitzer
Space Telescope observes in the infrared and has opened up a new universe
to us. Although it is not the first infrared space telescope, it is the largest
infrared space telescope ever launched so it has the greatest light-gathering
power and resolution of
any infrared telescope. A couple of examples of the power of infrared I use
in my class are the movie
of the dark globulae IC 1396 and the movie
of Herbig-Haro 46/47 on
the Spitzer
images site that
transitions from the visible light image to the near infrared to the mid-infrared.
Other movies contrasting
the views at visible with infrared of other places in the galaxy as well as
other galaxies are available there too.